1. Field of the Invention
The present invention relates to power supply switching circuits, and in particular to a device for automatically providing backup power to a load as a function of detected voltage levels.
2. Description of the Prior Art
A power supply is intended to provide power to a load. It can fail to provide adequate power for a number of reasons. For example, it may be intentionally or inadvertently turned off, or it may experience a short circuit or an open circuit such that its output power goes to zero, or the line connecting the power supply to the load may open. Similarly, a component failure within the power supply may cause the available power to be degraded below that level of power necessary to operate the load. In the event of such a failure, operation of the load is generally interrupted until the power is restored. However, a variety of loads can be envisioned which require uninterrupted power. Examples of such loads include clocks performing essential time keeping functions, volatile memories containing unique or critical data, and life support systems.
Backup power switching circuits are designed to switch the load to an auxiliary power supply in the event of failure of the primary power supply. Such circuits are interposed between the load and the primary power supply and optionally include the auxiliary power supply as part of the circuit. The auxiliary power supply, whether internal or external to the backup power switching circuit, is generally a lowpower battery having a limited ampere-hour useful life.
Previous backup power switching circuits have often employed discrete components which can consume significant power in their operation. The auxiliary power supply will thus be more rapidly drained and its useful life shortened. Additionally, the back-up circuit often tends to reduce the voltage provided to the load by the auxiliary power supply, which may further degrade operation of the essential features of the load.